The present invention is directed to a method and apparatus for capping headed stem fasteners, and more particularly, to a method and apparatus for controlling the nip profile and for increasing the nip length.
Various fasteners that releasably interengage with other articles are widely used as low cost garment fasteners, such as on disposable diapers. One type of headed stem fastener is the hook portion of a hook-and-loop fastener. Although the hook portion of a hook-and-loop fastener is typically designed to engage with a loop strip, the hook portion can be used by itself to become releasably fastened to fabrics that are easily penetrated by the hook. Another type of heated stem fastener that is particularly suited for this purpose is a mushroom-headed fastener, such as available under the product designation XMH-4152 from Minnesota Mining and Manufacturing Company of St. Paul, Minn. Mushroom-headed fasteners can be designated to become releasably fastened to burlap, Terri cloth, and tricot.
Stem fasteners are typically formed by capping polymeric stems extending distally from a backing layer. The precursor web containing the stems can be prepared according to a variety of techniques, such as disclosed in U.S. Pat. No. 4,290,174 (Kalleberg) and U.S. patent application Ser. No. 08/048,874 (Miller), entitled Mushroom-Type Hook Strip for a Mechanical Fastener (WO 94/23610).
FIGS. 1 and 3 are schematic illustrations of two commonly used methods for capping stems projecting upward from a precursor web. In the embodiment of FIG. 1, a precursor web 20 is fed through a gap in a nip 21 between two calender rolls 22 and 24. The heated calender roll 22 contacts a predetermined portion of a distal end 26 of the stems 28 projecting upward from a backing 30. The temperature of the heated calender roll 22 is maintained at a temperature that will readily deform the distal ends 26 under mechanical pressure in the nip 21.
Maintaining the distal ends 26 at this temperature allows melting and molecular disorientation of the stems 28. During such contact and/or upon subsequent cooling, a head 32 is formed on the distal ends 26. The heads 37 can be a variety of shapes, such as mushroom-shaped heads, xe2x80x9cumbrella,xe2x80x9d xe2x80x9cnail head,xe2x80x9d xe2x80x9cgolf teexe2x80x9d and xe2x80x9cJ-shaped.xe2x80x9d Mushroom shaped heads typically have a flat, planar or slightly convex upper surface and a maximum cross-section larger than the diameter of the stem immediately below the head (see FIGS. 8A and 8B).
The capping mechanism is generally a time-temperature-pressure phenomenon, although it is possible that some heat may be transmitted to the stems by convection. In practice, the height of the stems 28 and the finished height of the capped stem 32 are determined by the product design. The upper temperature at the roll 22 is generally limited to the temperature at which the polymer of the stems 28 sticks to the roll.
FIG. 2 is a diagram illustrating the size of the capping surface 34 (see FIG. 1) of a conventional calendering system. In FIG. 2, R is the radius of the heated roll, X is the distance over which the precursor web 20 is capped, t2 is the height of the capped stem 32, and t1 is the height of the stem 28. For a typical product, t2 is approximately 0.51 mm and t1 is approximately 0.74 mm. Using the following equation, the capping surface or distance 34 for a calender roll with a diameter of 45.7 cm (18 inches) is approximately 7.2 mm.   x  =                    2        ⁢        π        ⁢                  xe2x80x83                ⁢        R            360        ⁡          [                        cos          -                ⁢                  xe2x80x83                ⁢                  1          ⁡                      [                                          d                -                                  t                  1                                                            2                ⁢                R                                      ]                              ]      
FIG. 3 is a schematic illustration of an alternate method and apparatus for forming headed stems 42. The precursor web 20 is positioned so that a heated platen 40 is located above the stems 28. The heated platen 40 heats the air near the distal ends 26 of the stems 28 to cause the ends to soften by convection. The stems are deformed into generally hemispherical-shaped heads 42. In order to achieve controlled deformation of the distal ends 26, the temperature at which the heated platen 40 can be operated is limited by the polymer from which the stems 28 are constructed. Additionally, the ability to control the shapes of the heads 42 is limited.
The present invention is directed to a method and apparatus for capping headed stem fasteners. The present method and apparatus controls the nip profile and increases the nip length.
The present method of capping a headed stem fastener includes providing a precursor web having a backing with a rear surface, a front surface, and a multiplicity of polymeric stems projecting distally from the front surface of the backing. A heated member is positioned opposite a support surface to form a variable nip having a variable nip length. The support surface has a shape generally conforming to a contour of the heated member. The precursor web is fed along the length of the variable nip to compressively engage the polymeric stems between the heated member and the support surface so that distal ends of the polymeric stems are deformed.
A variety of nip profiles may be configured using the present heated member and support surface. The nip gap may decrease along the variable nip length. The nip gap may have a generally constant rate of decreases along the variable nip length. The nip gap may decrease more rapidly near a nip inlet than near a nip outlet or the nip gap may decrease more rapidly near a nip outlet than near a nip inlet. The nip gap may have a generally constant rate of decrease near a nip inlet and a nonuniform rate of increase near a nip outlet. The nip gap may remain constant along a portion of the variable nip length and vary elsewhere along the variable nip length.
The curved support structure forms a variable nip having a variable nip length that is significantly longer than can be achieved using a pair of rolls of a comparable diameter. Therefore, without changing the diameter of the heated roll, the present curved support structure permits the variable nip length to be increased. Since capping is generally a time-temperature-pressure phenomenon, for a given time, temperature and pressure, the line speed of the precursor web through the present variable nip is greater using the present curved support structure than using a conventional two roll nip. The combination of the present heated roll and curved support structure define a variable nip length preferably at least 1.25 times greater than the nip length defined by a pair of rolls having the same diameter as the heated roll, and more preferably at least 1.5 times greater, and most preferably at least 3.0 times greater.
The present invention is also directed to an apparatus for capping a precursor web. The precursor web has a multiplicity of polymeric stems projecting distally from a front surface of a backing. The apparatus includes a heated member opposite a support surface forming a variable nip having a variable nip length. The support surface has a shape generally conforming to a contour of the heated member. A feeding mechanism feeds the precursor film through the variable nip along the variable nip length to compressively engage the polymeric stems between the heated member and the support surface supports so that distal ends of the polymeric stems are deformed.
The heated member may be a heated roll opposite a curved support surface. The curved support surface preferably has a radius of curvature generally conforming to a radius of curvature of the heated roll. The support surface may be slid or rotated into engagement with the heat roll.
In an alternate embodiment, the heated member may be a heated belt opposite the support surface. The support surface may be a support belt. The shape of the heated belt may optionally be altered by a support roll or a curvilinear slide plate. The heated belt and the support surface define at least two tapered zones. Alternatively, the heated belt has a generally planar configuration.
The present invention includes moving the heated member at a rate greater than, less than or equal to a line speed of the precursor web through the variable nip. A low friction interface may be generated between the rear surface of the backing and the support surface. The low friction interface may be for example a fluid bearing or a low energy material on the support surface.
The distal ends of the polymeric stems may be deformed into a variety of shapes, such as mushroom-shaped heads, J-hooks and umbrella-shaped heads. The polymeric stems preferably project at a generally right angle from the front surface of the backing. The backing may be a polymeric film.
As used herein:
Variable nip refers to a nip formed by two or more members, one of which does not have a circular cross-section.
Variable nip length refers to the effective length of the variable nip in the machine direction..